Devices such as computers, along with overall computer systems, will occasionally fail, be damaged or be shut down due to internal fault conditions such as a failure of the system clock. However, a substantial percentage of computer failures, damage and downtime is due to fault conditions occurring externally of the computer. These latter fault conditions can be generated in the power supply for, and environment of, the computer. Typical external power supply fault conditions that can cause a computer failure or damage or downtime are out-of-sequence phases of a three-phase power supply, loss of a phase, line voltage high (overvoltage) or line voltage low (undervoltage), power line transients, and rapid successive interruptions of the power supply. Examples of external environmental fault conditions are overtemperature, excessive water accumulation on the computer floor and excessive smoke/particulates accumulation in the computer room air. Another important consideration in the protection problem is unauthorized access to the computer, which can result in the misuse or theft of data or damage to the computer by an unskilled operator.
The problem of protecting the computer against external fault conditions has become more acute with the development of more sophisticated computers which are designed and built to operate in specific optimum conditions. The same technology that has made minicomputers and microcomputers possible, has increased their sensitivity to even the smallest external disturbances. At the same time, electric power generation and other external support systems have become increasingly overburdened and unreliable. This protection problem for the computer user is further exacerbated by the fact that the computer manufacturers will not warrant against damage caused by poor power conditions or extremes in the environment or computer misuse.